Anticoagulant plasma protein; it is a cofactor to activated protein C in the degradation of coagulation factors Va and VIIIa. It helps to prevent coagulation and stimulating fibrinolysis. Defects in PROS1 are the cause of thrombophilia due to protein S deficiency, autosomal dominant (THPH5). A hemostatic disorder characterized by impaired regulation of blood coagulation and a tendency to recurrent venous thrombosis. However, many adults with heterozygous disease may be asymptomatic. Based on the plasma levels of total and free PROS1 antigen as well as the serine protease-activated protein C cofactor activity, three types of PROS1D have been described: type I, characterized by reduced total and free PROS1 antigen levels together with reduced anticoagulant activity; type III, in which only free PROS1 antigen and PROS1 activity levels are reduced; and the rare type II which is characterized by normal concentrations of both total and free PROS1 antigen, but low cofactor activity. Defects in PROS1 are the cause of thrombophilia due to protein S deficiency, autosomal recessive (THPH6). A very rare and severe hematologic disorder resulting in thrombosis and secondary hemorrhage usually beginning in early infancy. Some affected individuals develop neonatal purpura fulminans, multifocal thrombosis, or intracranial hemorrhage. Note: This description may include information from UniProtKB.